In the operation of gas turbines, nitrogen oxides (NOx) are often produced in the exhaust gas, with NOx including nitrogen monoxide (NO) and nitrogen dioxide (NO2). With regulations of NOx emissions becoming more stringent throughout the world, minimizing such emissions is an important design criterion. For example, the U.S. Environmental Protection Agency recently has implemented new regulations regarding hourly stack NO2 emissions during all periods of operation including startup.
One solution for reducing overall NOx emissions is the use of a selective catalyst reduction (“SCR”) system. Such a SCR system can be connected to the gas turbine exit via ducting. The SCR system adds a reductant, typically ammonia or urea, to the exhaust gas stream before passing the stream through a catalytic bed so as to absorb selectively the nitrogen oxides and the reducing agent. The absorbed components undergo a chemical reaction on the catalyst surface and the reaction products are desorbed. Specifically, the reductant reacts with the nitrogen oxides in the flow of the exhaust to form water and nitrogen. Catalysts that use other types of reductants are also known in the art.
The efficiency of the SCR system can depend in part on the temperature of the exhaust gas stream. The performance of the SCR system, however, also may be dependent in part on the level of NO2 as a percentage of the total NOx. For example, the preferred ratio may be about one-to-one for NO2-to-NO for catalysts used with ammonia or urea. Preferred ratios for other catalysts may range between zero-to-one and one-to-zero.
During some gas turbine operating conditions, the ratio of NO2-to-NO at the gas turbine exit can be quite low, with NO being the predominant constituent of total NOx. During other operating conditions, the ratio of NO2-to-NO can be quite high. The ratio of NO2-to-NO also can change as exhaust gases flow at relatively lower velocities in the ducting from the gas turbine exit toward the SCR, reacting with other exhaust gas constituents along the way. In fact, the conversion of NO to NO2 in combustion gases can be quite sensitive to temperature, the presence of carbon monoxide (CO), and the presence of unburned hydrocarbons (UHC's) due to uncombusted gas turbine fuels. The materials from which the ducting is constructed also can contribute to the conversion of NO to NO2. Conversion of NO to NO2 in gas turbine combustion streams can be undesirable because it can decrease NOx reduction efficiency in downstream NOx reduction equipment, including the SCR systems. Other components can also be present within the ducting that can influence the temperature of the combustion gases. These can include heat exchangers, duct burners and the like.
Conversion of NO to NO2 downstream of the gas turbine also can be influenced by the presence of CO catalysts. CO catalysts may be utilized downstream of the gas turbine yet upstream of the NOx reduction equipment to convert CO to CO2 so as to reduce CO emissions. Such CO catalysts also may promote conversion of NO to NO2. For example, a commercially-available CO catalyst can oxidize about 95% of CO to CO2 and also about 16% of NO to NO2 at 700 degrees Fahrenheit (about 371 degrees Celsius). Combustion gas temperatures at the CO catalyst can be somewhat lower than at the gas turbine exit. Such CO catalysts can subject the NOx reduction equipment to undesired higher ratios of NO2-to-NO.
There thus is a desire for improved methods and systems for the control of gas turbine engines in general and NOx reduction systems including SCR systems in specific so as to produce the minimum possible NO2 and/or to produce a desired ratio of NO2-to-NO so as to promote increased NOx reduction efficiency. Such improved methods and systems will promote reduced emissions without sacrificing overall gas turbine output and efficiency. Specifically, the improved methods and system described herein may be utilized to reduce NO-to-NO2 conversion upstream of the CO catalyst so as to ensure an acceptable NO2-to-NO ratio downstream of the CO catalyst and at the entry to the NOx reduction equipment.